For example, there has been developed an information recording medium, such as a multilayer type optical disc, for optically recording or reproducing an information signal (data) using a laser beam or the like. In such a multilayer type optical disc, if the interval between recording layers is large, a signal from the selected recording layer possibly deteriorates due to an influence of spherical aberration, so that the interval between recording layers tends to be narrowed. However, if the interval between recording layers is narrowed, because of so-called interlayer crosstalk, return light from the multilayer type optical disc includes not only a component of reflected light (hereinafter referred to as “signal light” as occasion demands) generated in a selected desired recording layer (hereinafter referred to as “one recording layer” as occasion demands) but also a component of reflected light (hereinafter referred to “stray light” as occasion demands) generated in another recording layer other than the one recording layer, at high level. Thus, a S/N ratio of the signal component of a reproduction signal or the like is possibly reduced, which possibly makes it hard to properly perform various controls, such as tracking control. Specifically, in general, it is known that the optical diameter of the signal light irradiated on a light receiving element (photo detector) (i.e. stability in an optical path within an optical pickup, and reliability in the control operation of the optical pickup) and the component of stray light have a relationship of tradeoff. Specifically, by increasing optical magnification and reducing an area of the light receiving element to be standardized, an influence of the stray light “Noise” to the signal level “Signal” is relatively reduced, by which the SN ratio (Signal to Noise Ratio) can be improved. However, the optical diameter of the signal light irradiated on the light receiving element is necessarily reduced, a position shift of the signal light is detected unnecessarily large, by the amount that the optical diameter of the signal light is reduced, if various signals, such as a tracking error signal, are generated in various divisional areas which constitute the light receiving element. Thus, it is necessary to adjust a mechanical, structural, and positional accuracy to be high in various actuators within the optical pickup, and to control the irradiation position of the signal light, highly accurately. That is, there is such a technical problem that the stability in the optical path within the optical pickup and the reliability in the control operation of the optical pickup are reduced.
Thus, for example, in a tracking method in the recording or reproduction of a dual-layer type Blu-ray disc, there has been suggested a technology for avoiding the stray light entering the light receiving element, by separating a push-pull signal from the signal light, using a hologram element. Alternatively, a patent document 1 discloses a technology of separating the reflected light from each recording layer highly accurately, using a difference in angle of the optical axis of the return light from each recording layer of the dual-layer type optical disc.
Patent document 1: Japanese Patent Application Laid Open NO. 2005-228436